I faced this strange problem on a friend’s computer which had a USB keyboard. The keyboard would not work in the GRUB boot menu! It would however work fine inside the BIOS setup and after booting into Ubuntu, it would work there too. Why was it not working only in GRUB?!
The solution turned out to be Legacy USB support. I enabled this in the computer’s BIOS and it started working in the GRUB boot menu! 😄